The present invention relates to inorganic composite materials of the class generally known as fiber-reinforced ceramic matrix composite materials. More specifically, the invention relates to inorganic reinforcing fibers for ceramic matrix composites incorporating B--N--C.sub.x coatings, such coatings imparting improved reinforcement capability to the fibers.
Fiber-reinforced ceramic matrix composites comprising glass-ceramic matrices are well known. U.S. Pat. No. 4,615,987 discloses silicon carbide fiber reinforced glass-ceramic composites wherein the matrix consists of an alkaline earth aluminosilicate glass-ceramic composition. Similar silicon-carbide-reinforced composites wherein the matrix consists of a barium-modified magnesium aluminosilicate glass-ceramic are reported in U.S. Pat. No. 4,589,900, while U.S. Pat. No. 4,755,489 discloses SiC-reinforced glass-ceramics wherein the glass-ceramic matrix contains excess Al.sub.2 O.sub.3 and consists predominantly of anorthite in combination with mullite or alumina.
Prospective uses for fiber-reinforced ceramic matrix composites such as described in these and other prior patents and literature include use as a structural element in high temperature environments such as heat engines. Thus the materials to be employed must not only exhibit good strength and toughness at ambient temperatures, but must also retain those desirable physical properties at the elevated temperatures encountered in the operating environment. Temperatures in the range of 700.degree.-1000.degree. C. and highly oxidizing conditions (due to the high-temperature activity of oxygen) are considered representative of such operating conditions.
An important problem which has been identified in silicon carbide reinforced ceramic matrix composites in this temperature regime is that of high temperature embrittlement. Hence, instead of exhibiting high toughness and strength after exposure to temperatures in the operation ranges desired, these materials become brittle and subject to sudden catastrophic breakage, rather than more gradual failure as typical of the original material. While the exact mechanism of embrittlement has not been fully explained, oxidative deterioration of the fiber-matrix interface is the probable cause. See, for example, R. L. Stewart et al., "Fracture of SiC Fiber/Glass-Ceramic Composites as a Function of Temperature," in Fracture Mechanics of Ceramics, R. C. Bradt et al. Ed., Volume 7, pages 33-51, Plenum (New York) 1986.
It is known to provide coatings on fiber reinforcement to be incorporated in composite materials in order to modify the behavior of the materials or the fibers therein. Specifically, the use of boron nitride coatings on silicon carbide fibers or other fibers to be incorporated in ceramic matrix materials such as SiO.sub.2, ZrO.sub.2, mullite, and cordierite is known in the art. Thus U.S. Pat. No. 4,642,271 suggests that the high-temperature strength and toughness of a composite comprising an SiO.sub.2 matrix and SiC reinforcing fibers may be significantly improved by coating the fibers with BN. This effect was, however, found to be dependent on the composition of both the fibers and the matrix, as the same fibers in other matrix materials did not always yield improved results. Thus, for example, BN-coated SiC fibers did not improve the characteristics of cordierite or ZrO.sub.2 composite ceramics.
Other coating systems and coating matrix combinations are also known. U.S. Pat. No. 4,276,804, for example, describes the application to carbon fibers of a metal oxide film intended to improve fiber adhesion and wetting by a molten metallic matrix material. U.S. Pat. No. 4,397,901 describes a composite article and method for making it wherein a woven or non-woven fiber substrate, typically composed of carbon fibers, is provided with successive coatings of pyrolytic carbon, diffused silicon, and silicon carbide to provide a composite article resistant to corrosive conditions. U.S. Pat. No. 4,405,685 describes a similar coating system for carbon fibers wherein an inner coating consisting of a mixture of carbon and a selected metal carbide, in combination with an outer coating consisting solely of the metal carbide, are described. This dual coating system is intended to provide enhanced fiber protection for fibers to be embedded in ceramic or particularly metal matrix materials.
U.S. Pat. No. 4,481,257 discloses silicon carbide monofilaments coated with boron or boron carbide. These filaments exhibit improved strength and bonding when used with metal or epoxy matrix materials. U.S. Pat. No. 4,485,179 describes the use, in a ceramic matrix composite comprising silicon carbide fibers, of an agent added to the matrix to reduce interaction with the silicon carbide fibers. Tantalum or niobium compounds are useful for this purpose.
While the foregoing patents and literature indicate a general interest in the development of coatings for fibers to be employed for the reinforcement of composite glass, metal and ceramic materials, the problem of embrittlement of ceramic matrix composites comprising silicon carbide or other fibers remains.
It is therefore a principal object of the present invention to provide a fiber-reinforced ceramic matrix composite comprising inorganic reinforcing fibers which exhibits improved resistance to embrittlement under adverse high temperature conditions.
It is a further object of the invention to provide a method for making silicon carbide-reinforced glass-ceramic matrix composites which provides products of improved strength and/or toughness at high temperatures.
Other objects and advantages of the invention will become apparent from the following description thereof.